1. Field of the Invention
This invention relates generally to thermal insulation, and more specifically to an improved thermal insulation including relatively long non-combustible metallic filaments randomly interlocked to form a wool.
2. Description of the Prior Art
The prior art includes numerous types of insulations, each having particular physical and thermal characteristics, thereby rending these different materials particularly adaptable to a given application.
For example, U.S. Pat. No. 273,688 of Mar. 6, 1883 describes a metal mesh positioned adjacent to a conduit so as to keep a thermally non-conducting plastic mass or cement spaced from the conduit thus forming a dead air space between the conduit and the non-conducting mass.
U.S. Pat. No. 2,179,057 of Nov. 7, 1939 describes the use of an asbestos paper having nibs on it. Nibbed sheets are laid up such that each nibbed section forms an air cell. Also disclosed in the use of aluminum foil laminated to the asbestos paper nibbed sheets.
U.S. Pat. No. 2,514,170 of July 4, 1950 relates to a thermal insulation for jet engines and the like in which various forms are illustrated. In one form, an open metal mesh supports a pocket filled with asbestos and the like in spaced relation to a tubular member. In another form, a radiation shield in the form of a foil is used, the outer surface being an asbestos cloth. Several other forms of thermal insulation are shown, each using wire mesh members.
U.S. Pat. No. 3,007,596 of Nov. 7, 1961 describes an insulation made up of alternate layers of radiation shield material and an insulation material such as glass wool.
For high temperature use, e.g. 200.degree. to 4000.degree.F. various materials are commercially available, but are nonmetallic in nature. Typical are aluminum silicate powders, silica fibers, zirconium oxide powders, potassium titanate, glass fibers, aluminum fibers, expanded pearlite, collodial silica and silica aerogels. These materials may be used as ceramic foams (silica carbide, aluminum zirconia and silica) with organic binders such as an epoxy resin. Multilayer composites of fiber glass or foam blankets are also available.
While certain of these materials have low thermal conductivity; e.g. about 0.2 btu/hr.-ft.sup.2 -.degree.F/inch for mixture of powders and polymer binders, they also have high densities, for example 20 to 30 lbs/ft.sup.3, although some commercially available materials, in the form of batting, have densities as low as 3.5 lbs/ft.sup.3.
As a general rule, these materials have low compressive strengths, i.e. about 3600 lb/in.sup.2 at densities of 30 lbs/ft.sup.3. Moreover, there are serious limitations on the environments in which they can be used, e.g. chemical, structural etc. In some cases, there may be erosion problems or moisture affinity or affinity for certain gases which adversely affect performance of the insulation system. Weight and thickness may also present some problems especially when the insulation is to be used in the aircraft or aerospace fields, in that in order to provide adequate protection, the insulation may be too bulky, too heavy or inherently incapable of providing the structural strengths needed for the severe physical environment in which the insulation is to be used.
For example, in helicopter rotor blades which are designed to provide a gas duct for engine exhaust, it is desirable to insulate the interior surface of the rotor blade from the exhaust gases. In such an application, the exhaust gases may be as high as 1000.degree.F. at a pressure of 40 pounds per square inch absolute with an internal flow Mach number of 0.45. From these extreme conditions inside the rotor blade, it is desirable to limit the temperature of the rotor blade to a value such as 180.degree.F. for several reasons. At a temperatures of 180.degree.F., the blade can be formed from conventional materials. This would be advantageous at least from economical and structural standpoints. Also, by decreasing the temperature of the blade, the infrared signature is lowered so that the helicopter cannot be as easily detected at night. This is desirable in military applications.
In such a severe environment an insulation having physical characteristics for withstanding a temperature such as 1000.degree.F. and thermal characteristics including a thermal conductivity as low as air would be quite desirable. The insulation systems presently available which have these characteristics unfortunately are deficient structurally. For example, some prior art and presently available insulation systems include chaalk-like materials which have a tendency to crumble and spall. These characteristics have made such insulation systems not well suited for use in severe environments such as a rotor blade where extreme vibration and centrifugal force loads are present. The chalk-like materials are moisture absorbent and chemically reactive with exhaust products. High temperatures have vaporized the moisture and altered the composition of the insulation itself to produce undesirable gases within the insulation. This is typically referred to as outgassing. Furthermore, in proximity to the high velocity fluids, the chalk-like materials can be expected to erode so as to decrease the thickness of the insulation. A reduced thickness would be detrimental not only to the structural characteristics but also to the thermal characteristics of the insulation.
By way of example, ducts for transporting hot fluids, such as gases, have typically been insulated by applying an insulation blanket to the outer surface of the duct. Although the insulation blanket has provided a cooler outer surface, it has been necessary to form the duct from materials which can withstand the temperature of the fluid. In most cases these ducts have been formed from high density metals which have made them particularly heavy for aircraft applications. These metals have not been thermally relieved so that thermal expansion has been a problem in some applications.
Many of the advanced insulation systems having a thermal conductivity substantially equal to that of air have included chalk-like materials which have crumbled and spalled, especially in severe environments wherein vibration and significant forces have been encountered. These insulation systems have also been water absorbent so that outgassing has been particularly critical. To compound these problems, these insulation systems have eroded so that their insulating characteristics have been significantly degraded.
Insulation blankets have also been used to provide garments, such as trousers, shirts, jackets and hats, for use by those who are subjected to environments having elevated temperatures. For example, fire fighters typically wear insulated garments to increase their comfort in proximity to a fire. Other persons, such as race car drivers and pilots, perform functions in environments wherein fires are more probable and can be particularly severe.
A garment commonly in use is made from an insulation fabric formed from nylon and treated in accordance with a process patented by DuPont. This fabric is commonly sold under the tradename "NOMEX". Although this fabric is nonirritating next to the skin and has characteristics for inhibiting the propagation of flame, it tends to decompose and otherwise degrade at temperatures above 750.degree.F.
Insulation garments have also been formed from fabrics which have included asbestos. Asbestos has been undesirable since its fibers, if inhaled, can impregnate the lungs and cause disease. These asbestos garments have also been particularly heavy and have tended to crack when creased. They have been moisture absorbent and have tended to react chemically with some elements.
It has been desirable to provide fire walls between fire-producing environments and environments which are preferably protected from fire. For example, fire walls have been provided between wing fuel tanks and the fuselage of an aircraft. The specifications of the Federal Aeronautics Administration require that a fire wall be capable of withstanding temperatures of 1800.degree.F. for a period of 10 minutes. These stringent criteria have typically been met by providing solid metal plates typically formed from steel having a density of 494 pounds per cubic foot. Due to this high density, fire walls have been extremely heavy, a characteristic which is particularly undesirable in aircraft applications.